The present invention relates generally to two-way communication networks of the type used in cable television systems and particularly concerns techniques for providing downstream acknowledgments of subscriber upstream messages.
Cable television systems utilize a central provider of program information which services a large number of end users generally referred to as subscribers. The central provider portion of the cable television system, usually called the "headend", provides a plurality of program information as well as other information to the subscribers via a multiple branch distribution network which may define several tiers of distribution facilities.
In addition to actual programming information, the cable television system is required to carry additional management and operating data, such as conditional access data, provided by the headend to the large number of end users or subscribers. Communication from the headend to the subscribers is generally referred to as "downstream" communication. In some cable television systems referred to as one-way, all information and data is transferred downstream. In other cable television systems referred to as two-way systems, communication is also provided from the various subscribers throughout the network to the headend in what is referred to as "upstream" communication.
Upstream communication in a cable television system is normally provided using an out-of-band carrier, usually between five and thirty megahertz, which is modulated with upstream data at the subscriber decoder and transmitted to the headend. Examples of subscriber originated upstream messages may include program purchasing requests, opinion poll responses, and subscriber terminal status information. Some of these messages, e.g. program requests, may require downstream acknowledgment and authorization while others, e.g. terminal status information, may be initiated in response to an appropriate downstream command.
Upstream communications are typically effected using either a contention protocol as disclosed in U.S. Pat. Nos. 4,528,663 and 4,553,161 wherein the subscriber's contend for access to the upstream channel or by providing each subscriber with guaranteed access to a respective relatively small portion of the upstream spectrum. Depending on the type and quantity of usage, one protocol may be more advantageous than the other, or a mixture of both may provide the most effective performance. In either case, upstream messages are transmitted in respective time slots which may be assigned to individual subscribers on a reserved basis to guarantee access to the upstream channel or which multiple subscribers may use on a contention basis. Acknowledgment by the central facility of successfully received upstream subscriber messages is normally effected using dedicated downstream acknowledgment messages. Such acknowledgment messages may, for example, be multiplexed as auxiliary data packets in a downstream transport bitstream and separated therefrom in the subscriber's terminal on the basis of their unique header identification code. This approach is wasteful of downstream overhead and is therefore not considered desirable. This approach suffers the further disadvantage that the downstream acknowledgment messages are substantially displaced in time from the downstream messages providing subscriber authorization of a requested program or service since the two are transmitted in separate downstream packets.
It is therefore a basic object of the present invention to provide an improved two-way data transmission system for a subscription service such as a two-way cable television network.
It is a more specific object of the invention to provide a two-way data transmission system which provides downstream acknowledgment of upstream subscriber messages using a minimum amount of overhead.
It is a further object of the invention to provide a two-way data transmission system in which disjunctive downstream responses to upstream messages are minimized.